Ratio of the Prompt Fission Neutron Spectrum of239Pu to That of235U

The prompt fission neutron spectrum resulting from 239Pu fission induced by ∼0.55-MeV incident neutrons is measured from 1.0 to 10.0 MeV relative to that of 235U fission induced by the same incident energy neutrons. The measurements employ the time-of-flight technique. Energy-dependent ratios of the two spectra are deduced from the measured values. The experimentally derived ratio results are compared with those calculated from ENDF/B-V (revision 2) and with results of recent microscopic measurements. Using the ENDF/B-V 235U Watt parameters for the 235U spectrum, the experimental measurements imply a ratio of average fission spectrum energies of 239Pu/235U = 1.045 ± 0.003, compared to the value 1.046 calculated from ENDF/B-V (revision 2).